Dom, this may be a long shot, but here are my ideas. Assuming the volumes were accurately measured I don't see how you could miss the OG by that much for an extract recipe. So I'm wondering at what temperature you measured your OG. For example, you say you measured an OG of 1.045. But, if the temperature of the wort was 75*C when you took the reading the actual OG would be 1.069 after correcting for temperature. If you added the top off water before taking the SG reading it could be that it was not completely mixed into the wort and your sample was diluted.